There are known systems for determining the position or motion of a movable device, such as a stylus, a pen, a whiteboard marker, or other pointing device over a plane, which use signals transmitted through a medium, such as air, between one or more transmitters and one or more receivers, wherein the position is determined by proper processing of received signals; such systems are sometimes also referred to as active tracking systems or as location transcription systems and will collectively be referred to herein as locating systems. In one version of a locating system, depicted schematically in FIG. 1A, a transmitter 1 that is attached to a pointing device 2 emits a signal of repeated ultrasonic pulses and this signal is received by two or more receivers at known locations, e.g., attached to a stationary frame near an active substantially planar area. FIG. 1A shows two ultrasonic receivers 3 attached to a frame 5. The pointing device 2 also emits infrared pulses that are received by an infrared detector 4 on the frame 5. The relative times of arrival of the signals at each receiver are detected and from these values the distances from the transmitter 1 to the receivers 3 and 4 are determined and hence, by triangulation, also the position of the pointing device 2 with respect to the stationary frame. Thus, an active area 8 is defined wherein the location system can determine the location of the pointing device 2. A transmitter 9 transmits the so-determined positions, e.g., to a matching receiver 10 on a computer such as a laptop computer 11. Transcriptions of a set of locations, e.g., a line 12, is sent and may be stored in the computer 11.
FIG. 1B shows an example of the pointing device 2, that may include a tip 15 that may be a marking element. The pointing device includes a body 13 and a pressure sensitive switch 14 that starts transmission of the signals from the transmitters when the tip is pressed against the planar surface. One or more buttons 16 may be included in the pointing device to provide indications to the receiver array containing receivers 3 and 4.
U.S. Pat. No. 6,335,723 to Wood, et al., entitled “TRANSMITTER PEN LOCATION SYSTEM,” assigned to the assignee of the present invention and incorporated herein by reference, discloses one such system for locating the position of a pen. It is noted that, in addition to a relatively slow-propagation signal, typically an ultrasonic signal, Wood, et al. also describes a version that includes on the pointing device a transmitter such that there also is transmitted and received a fast-propagation signal, typically an infrared signal, which serves to provide the ultrasonic receivers with a time base for calculating the respective propagation times of the slow signal. Both transmitted signals are structured as a mutually synchronized train of pulses. It is further noted that in an alternative embodiment, any of the signals may also serve to carry supplementary information, such as pen color, and/or other pen parameters.
U.S. Pat. No. 6,414,673 to Wood, et al., entitled “TRANSMITTER PEN LOCATION SYSTEM,” assigned to the assignee of the present invention and incorporated herein by reference, discloses another such system. Here, basically, the directions rather than the times of arrival of the transmitted signal at each receiver are detected and hence the location of the pen is calculated. In one embodiment, the time of arrival at a receiver is also detected. U.S. Pat. No. 6,184,873 to Ward, et al., entitled “PEN POSITIONING SYSTEM,” assigned to the assignee of the present invention and incorporated herein by reference, discloses a variation of the aforementioned systems, wherein there are two output elements attached to the pen, at given different distances from its tip, each transmitting at a unique ultrasonic frequency. The two frequencies are processed separately at the receivers, to determine the respective positions of the output elements; from these, the position of the tip of the pen is calculated.
Further variations of the aforementioned systems are possible. For example, there may be a single receiver, to determine a position along a single axis, or the number of receivers may be greater than two—to increase the accuracy of triangulation, to determine the position along three dimensions, or to increase the active area. As another example, one or more receivers may be attached to the movable device while transmitters are attached to the stationary frame. As yet another example, the medium, which in the aforementioned systems is air, may be a vacuum or may consist of any other substance, whether gaseous, liquid or solid; concomitantly, the signaling modality, besides being acoustic or electromagnetic, as in the aforementioned systems, may be any other type, such as surface acoustic waves. The pointing device itself may be of any shape and may serve any purpose in addition to just being locatable and its motion may be effected by a human operator or by a machine. The pointing device typically is movable.
FIG. 1C shows a typical functional block diagram of the sensor array that includes sensors 3 and 4 and that includes processing of the signals received by the sensors. A signal conditioner (17A, 17B, 17C) includes filtering of signals that are out of the expected frequency range, and includes anti-alias filtering. The pen 2 simultaneously transmits an ultrasound pulse and an infrared (IR) pulse. The IR pulse is assumed to travel much faster than the ultrasound pulse, and hence is received first at the IR receiver 4. An analog-to-digital converter (ADC) (18A, 18B) initially converts the infrared pulses and these digitized infrared signals are input serially to a processor, in one embodiment, a DSP device 19. In particular, the data is input to the memory of the DSP device 19. To reduce costs of additional ADCs, a switch then switches the input of the ADC to receive signals from one of the ultrasound receivers. Another ADC also receives signals from the second ultrasound receiver. Thus, after the IR pulse is received, digitized ultrasound signals are received, digitized, and input to the memory of the DSP device 19 for further processing.
In one embodiment, the program for such processing is kept in a flash memory coupled to the bus of the DSP device. The processing determines the time of arrival of the ultrasonic pulses received at the two receivers relative to that of the infrared signal. From these times of arrival and the known positions of the ultrasound receivers, the DSP device calculates the location of the transmitter at the time the ultrasound pulses were transmitted.
In one embodiment, the location information is transmitted, e.g., via Bluetooth technology or a USB cable, to another device such as a computer 11.
Common to all such systems, which the present invention addresses, is the presence of at least one transmitter and at least one receiver, at least one of which is attached to a movable device; any transmitter transmits through the medium at least one useful signal, which is received by at least one receiver and subsequently processed; the processing of one or more of the received signals leads to a determination of the current position or velocity of the movable device along at least one dimension. The useful signal may be either in a slow propagating mode, such as acoustic (usually ultrasonic) waves, serving to manifest propagation time that is proportional to, and thus indicative of, the distance traveled in the medium, or it may be in a fast propagating mode, such as electromagnetic waves (usually in the IR range), serving, for example, to provide time reference; in either role it will also be referred to herein as a locating signal, though it may optionally have other information encoded thereto. A locating signal is generally characterized by a given carrier frequency, as is known in signaling practice.
It is often the case that there is an interfering signal present in the medium or transmitted therethrough, such a signal emanating, e.g., from a source other than any of the system's transmitters, and such a signal received by any of the receivers in addition to the useful signal. The interfering signal may, for example, be electromagnetic induction from power lines and devices, light from high-frequency lighting devices or an ultrasonic signal from another source, such as a motion detector. Such interfering signals may degrade the results of the corresponding processing, possibly causing an error in the determined position or velocity or even making such determination altogether impracticable. Generally, locating and tracking systems require a relatively high degree of accuracy and resolution—typically 1:1000 or better—and therefore even relatively low levels of interfering signals may be deleterious. Interfering signals may be regarded as noise. In this description, noise, interference, interfering noise, and so forth are all used to mean the signal or signals interfering with the locating signals.
Now, if the interfering signal is clearly distinguishable from the useful signal—for example, by having all frequencies substantially different from the useful signal's carrier frequency or by occurring within time periods distinct from those in which the useful signal occurs—then the component of the received signal due to the interfering signal may be removed or sufficiently reduced, using filtering techniques known in the art. If, however, the interfering signal has frequency components close, or identical, to the useful signal's frequency and if it occurs substantially during time periods at which the useful signal occurs, no such filtering is effective for such components. It thus is desirable and would be useful to have a method and apparatus to reduce interfering components in the received signal in a locating system especially in such cases as last discussed.
The inventors have found, for example, that for infrared detectors, fluorescent lights often emit interference in the same frequency range as the infrared location signals used in location determining systems. Furthermore, ultrasound motion detectors often produce ultrasound signals that are in the same frequency range as the ultrasound location signals used in location determining systems, and that are so strong as to reduce the accuracy of such systems, possibly even rendering the location determining system unusable in the presence of the interference.
Thus there is a need in the art for a method for reducing the amount of interfering noise in locating signals used in location determining systems.